1. Field of the Invention
The present invention relates to an acceleration sensor for sensing earthquake vibrations and others, and particularly to such a sensor which detects accelerations in many directions on a two-dimensional plane at approximately the same sensitivity and may be built in a gas or other flow meter so as to shut off the valve of a gas pipe.
2. Description of Related Art
As a conventional type acceleration sensor, an acceleration sensor disclosed in Japanese examined utility model publication No. Sho 50-19154 and a dynamic quantity sensor for detecting dynamic quantity such as angular velocity and acceleration which is disclosed in Japanese unexamined patent publications No. Hei 6-123631 and No. Hei 6-123632 are well-known.
The acceleration sensor disclosed in Japanese examined utility model publication No. Sho 50-19154 is formed by assembling an anchor displaced depending upon applied acceleration, a beam for supporting the anchor and a fixed electrode for detecting contact with the anchor which are respectively made of metallic material by machining. However, this acceleration sensor needs a troublesome assembly work of each member, size reduction is difficult and it is unsuitable for mass production.
A semiconductor dynamic quantity sensor disclosed in Japanese unexamined patent publications No. Hei 6-123631 and No. Hei 6-123632 can detect dynamic quantity of the low level of acceleration and a frequency with high precision, difficulty in assembling each member is obviated because each member is formed in the same semiconductor process and it is regarded as promising because it can be manufactured at a low price and is suitable for mass production.
This semiconductor dynamic quantity sensor detects changes of capacitance between movable electrodes which are moved together with an anchor supported by four beams so that it can be moved in two directions of X and Y, and fixed electrodes which are provided on the side of a substrate so as to measure acceleration. This sensor can detect acceleration in all directions on an X-Y plane by calculating the magnitude of acceleration detected in each direction of X and Y.
However, an arithmetic circuit for processing a signal synthesization based upon each result of detection in the directions of X and Y is required to detect the magnitude of acceleration in the diagonal direction on the X-Y plane, thus complicating signal processing circuit.